1. Field of the Invention
The present invention relates to an optical coupling device including a light emitting element and a light receiving element and particularly to an optical coupling device capable of controlling light emission intensity from the light emitting element.
2. Description of Related Art
A photocoupler is used in various locations such as factories, plants, and electrical household appliances for the purpose of eliminating a large common mode noise and ensuring safety. The photocoupler has a configuration where a light emitting element such as a light emitting diode (LED) which emits light in accordance with an input signal and a light receiving element such as a photodiode which outputs a current in accordance with an incident light intensity are optically coupled, and its input and output are electrically disconnected.
A large common mode noise occurs often in the sites where high-power equipment and high-sensitivity electronic equipment coexist with a difference in power supply voltage exceeding 1000V, such as factories and plants. There has been a recent demand for controlling AC servos and inverters which are used in factories or the like highly accurately. To meet this demand, a photocoupler is required to exhibit higher common-mode rejection (CMR) and lower pulse width distortion, and various techniques have been proposed therefor, including Japanese Unexamined Patent Application Publication No. 2002-353495 (Shimizu), for example.
FIG. 5 illustrates the configuration of a photocoupler 1 taught by Shimizu. The photocoupler 1 converts an electrical signal which is input through an input terminal 2 into an optical signal in a light emitter 3, then converts the optical signal back to an electrical signal in a light receiver 4 on the output side, and finally outputs the electrical signal through an output terminal 5. A light emitting element on the light emitter 3, such as a LED, and a light receiving element on the light receiver 4, such as a photodiode, are disposed face to face each other in close proximity. A light transmitting resin 6 having a prescribed dielectric constant is filled in the gap between the light emitting element and the light receiving element. The light emitter 3, the light receiver 4, and the light transmitting resin 6 are sealed in one package by a light shielding resin 7. The input-side circuit and the output-side circuit are thereby electrically shielded from one another, thereby enabling signal transmission under the condition where devices are electrically disconnected.
FIG. 6 illustrates the configuration of the light receiver 4 taught by Shimizu. As shown in FIG. 6, the light receiver 4 includes a photodiode D1 and a dummy photodiode D2 which are arranged adjacent to each other. The photodiode D1 receives an optical signal from the light emitting element. A light receiving surface of the dummy photodiode D2 is shielded from light by a cathode electrode. The output currents from the photodiodes D1 and D2 are respectively converted into voltages by current-voltage conversion amplifiers A1 and A2. The voltages converted by the current-voltage conversion amplifiers A1 and A2 are then compared with each other by a hysteresis comparator 8, and finally output as waveform-shaped pulses. This enables improvement in a common-mode rejection ratio (CMRR).
The light receiver 4 further includes impedance variable circuits Z1 and Z2 in which impedance decreases as the level of a current input to the amplifier increases. In such a configuration, if a current signal Ipd is Low, the impedance of the impedance variable circuit Z1 increases. This reduces the bandwidth of the amplifier to suppress the high-frequency component of a noise and substantially equalizes the values of Z1 and Z2. The characteristics of the current-voltage conversion amplifiers A1 and A2 are thereby aligned to improve the CMRR. On the other hand, although the reduction of the bandwidth of the amplifier causes rounding of the received waveform, it is output because a threshold level of the hysteresis comparator 8 is constant. The pulse waveform is thereby distorted, resulting in large pulse width distortion. If, on the other hand, the current signal Ipd is High, the impedance of the impedance variable circuit Z1 decreases. This enlarges the bandwidth of the amplifier to achieve low pulse width distortion. Instead, the high frequency component of a noise cannot be suppressed, and the values of Z1 and Z2 thereby differ. The characteristics of the current-voltage conversion amplifiers A1 and A2 are thereby unbalanced to deteriorate the CMRR.
The light emission intensity of LED varies by manufacturing fluctuation, changes over time, ambient temperature variation, and so on. The non-uniform light emission intensity causes fluctuation in the current signal Ipd which is detected by the light receiver 4. In Japanese Unexamined Patent Application Publication No. 9-172225 (Asami et. al.), a technique of monitoring a part of optical power and stabilizing the optical power is disclosed. However, it contains no suggestion for applying this technique to a photocoupler.
According to the related arts, the CMRR and the pulse width distortion are in the relation of trade off each other with respect to a change in the current signal Ipd, thus failing to achieve both high CMR and low pulse width distortion. As a photocoupler operates at higher speed, the restriction of pulse width distortion becomes more strict, i.e. from 100 ns or lower to 10 ns or lower. It is difficult for the techniques according to the related arts to satisfy such restriction.
In view of the foregoing, there is an increasing need for an optical coupling device capable of controlling the light emission intensity from a light emitting element and achieving both high CMR and low pulse width distortion.